1. Field of the Invention
The present invention relates to a device for supplying a load object with direct voltage pulses with a variable frequency from a direct voltage source, said device comprising a pulse-generating member for connection between the direct-voltage source and the load object and arranged to deliver direct voltage pulses with a variable frequency to the load object, which member is switchable between pulsing in groups and symmetrical pulsing, and, in the case of pulsing in groups, delivers pulses in groups of two pulses with a constant time interval between the two pulses of each group.
2. The Prior Art
In devices of this kind, the current flowing from the direct voltage source consists of periodic current pulses, which means that the current has a DC component as well as an AC component consisting of a fundamental tone and a number of harmonics. In certain fields of application, an AC component with a certain frequency may be harmful. This is the case, for example, in rail vehicle operation, where the engines of the vehicle are supplied from a live contact line or contact rail by way of one or more direct voltage convertors placed in the vehicle, which are used for the control of the engines. Usually signal systems are used in this case which transmit information to the vehicle, preferably regarding the allowed speed, with the help of a coded or modulated carrier frequency signal transferred, for example, by way of the rail. In a typical case, the carrier frequency is 75 Hz. It is of great importance that the signal system will not be disturbed by the load current of the vehicle. It is possible, in a manner known per se, to control the direct voltage convertor so that the current flowing form the source consists of pulse groups with a definite number of pulses in each group, with a constant time interval between the pulses in the group and with a variable time interval between the groups. In this way, a certain discrete frequency (the above-mentioned carrier frequency) can be completely eliminated from the current flowing from the source.
According to a known control principle, the current pulsing from the source consists of pulse groups with two pulses in each group, the time interval between which is constant. The time interval between two consecutive groups may be varied so that the average value of the direct voltage applied to the object may be controlled.
The AC component of the current contains a fundamental tone with a frequency which is the same as the frequency with which the pulse groups appear and in addition there is a great number of harmonics. The frequency f.sub.0 = (1/2T.sub.1), where T.sub.1 is the time between the two pulses of a group, will be missing from these harmonics, and, by choosing the time between successive pulses of each group suitably a certain frequency may be completely avoided.
In the known system, in order to increase the average value of the voltage on the load object, the time between the groups is decreased. When the time between pulses is equal to that between groups, there is so-called symmetrical pulsing. The fundamental tone has completely disappeared. When the load voltage is further increased, the time interval between the pulses is reduced while maintaining symmetrical control.
A problem in devices of this kind is that frequently it is not sufficient to eliminate only one single discrete frequency. In the signal system for vehicles described above, a band pass filter tuned to the carrier frequency is usually included, which band pass filter is intended to let through the carrier frequency only. To be able to transmit information by coding the carrier frequency, the band pass filter must have a certain band width. This means that, even if the carrier frequency itself is eliminated by pulsing in groups, the fundamental tone or the harmonics may come within the pass band of the filter. Particularly the fundamental tone may find its way into the pass band of the filter and seriously disturb the signal system when the group frequency approaches f.sub.0.
The disturbances can be particularly serious when several direct voltage convertors, independent of each other, are working in parallel and are controlled to have the same voltage, which is the case, for example, in a train consisting of several motor coaches. Disturbances with a frequency near the carrier frequency will give rise to beating, where the maximum amplitude of the disturbances is determined by the sum of the amplitudes of the disturbing current components from the different coaches. Furthermore, the heating may be interpreted by the signal system as a modulation or coding of the carrier frequency, and therefore, for example, may cause the vehicle to be allowed to continue running in spite of the fact that a signal for such an allowance has never been sent to the vehicle.